Ensure compatibility with Boost < 1.60

This removes an unnecessary reference to
<boost/math/special_functions/relative_difference.hpp>.
6 jobs for master in 229 minutes and 25 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Test
passed #2888
amd64
job_clang_amd64

03:49:24

passed #2892
amd64
job_clang_amd64_nosh

03:20:45

passed #2890
amd64
job_clang_py2_amd64

03:44:03

passed #2887
amd64
job_gcc_amd64

03:20:18

passed #2891
amd64
job_gcc_amd64_nosh

02:51:50

passed #2889
amd64
job_gcc_py2_amd64

03:10:37